In recent years, the increasing demand for information capacity of optical fiber systems has made telecommunications manufacturers develop methods and devices for in particular wavelength division multiplexing (WDM). For these systems, the signal information is transmitted on distinct channels of optical light. The signal information can comprise a plurality of logical signal channels, and each signal channel may, in turn, include both time division multiplexed (TDM) and space division multiplexed (SDM) components, space division multiplexing (SDM) meaning that separate fibers are used for different parts of a message transferred in a logical channel.
The preferred wavelengths for most telecommunication optical fiber systems are in the infrared part of the spectrum, around 1500 nm, due mostly to the low attenuation and the low signal pulse broadening when transmitting signals on optical fibers in this region, but also because of the availability of suitable light sources and detectors. In particular for WDM, another advantage here is the availability of various types of optical amplifiers. These are necessary since each wavelength channel carries only a small portion of the total power of light propagating in the fiber and thus needs to be amplified to compensate for optical losses in the fiber link, in order to get a sufficient signal-to-noise ratio at the receiver end.
There are various designs of optical amplifiers. The most important ones for telecommunication applications include erbium-doped fiber amplifiers (EDFA), semiconductor optical amplifiers (SOA), Raman amplifiers (RA), and optical parametric amplifiers (OPA). These amplifiers have specific advantages and disadvantages.
Raman amplifiers are of a special interest due to some important features. Such amplifiers differ from the others mentioned above in that the gain thereof is distributed over a given length of the optical fiber used, the Raman fiber. The Raman fiber is connected in series with the ordinary transmission fiber, preferably near the transmitting light source. The power necessary for the amplification is delivered by pumping light from at least one separate pump light source. The maximum value and the shape of the Raman gain depend on the wavelength of the light emitted by the pump light source, rather than on the fiber itself. Usually, injection of pump power takes place near the input end of the Raman fiber, using, e.g., a fiber-optical coupling device. Pump light of different wavelengths from several distinct pump light sources can be injected in parallel in order to achieve a desired shape of the Raman gain, see the published International patent application WO 00/49721. A problem with this pumping method is that nonlinear interaction may take place between the various wavelength contributions. Also, the need for several pump light source and the intricate control thereof make such amplifiers complicated and costly.